Psychrometric Properties Calculator
An essential tool for HVAC engineers to determine state points of moist air. Calculate Dew Point, Wet Bulb, Enthalpy, Humidity Ratio, and more. Accurately corrects for Altitude/Pressure using ASHRAE Fundamentals logic.
Psychrometric Report
Psychrometric Chart Location (Schematic)
State: Undefined
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The Ultimate Guide to Psychrometrics
What is Psychrometrics?
Psychrometrics is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures. In the context of HVAC (Heating, Ventilation, and Air Conditioning), it specifically refers to the study of Moist Air—a mixture of dry air and water vapor.
Understanding these properties is the foundation of designing any air conditioning system. Whether you are cooling a data center, drying pharmaceutical powders, or simply keeping a home comfortable, you are manipulating the state points on the Psychrometric Chart.
1. The "Big Three" Temperatures
Confusing these three temperatures is the most common mistake for students and junior engineers. They are distinct properties that define the state of the air.
| Property | Symbol | Definition | Measurement |
|---|---|---|---|
| Dry Bulb | $T_{db}$ | The true thermodynamic temperature of the air. It is a measure of sensible heat only. | Standard thermometer or thermocouple. |
| Wet Bulb | $T_{wb}$ | The lowest temperature that can be reached by evaporating water into the air adiabatically. It indicates the total heat (enthalpy) of the air. | Thermometer with a wet wick over the bulb, exposed to moving air. |
| Dew Point | $T_{dp}$ | The temperature at which moisture begins to condense out of the air. It is a direct measure of the absolute amount of water in the air. | Chilled mirror hygrometer. |
Why Wet Bulb Matters
Wet bulb temperature is critical for Cooling Towers and Evaporative Coolers. A cooling tower can never cool water below the ambient wet bulb temperature. If the $T_{db}$ is 35°C but the $T_{wb}$ is 28°C, your cooling tower water will likely be around 31°C-32°C. Ignoring $T_{wb}$ leads to undersized cooling equipment.
2. Enthalpy ($h$): The Energy Content
Enthalpy is the total energy content of the moist air, usually expressed in kJ/kg of dry air. It is the sum of two components:
- Sensible Heat: The energy associated with the temperature of the air and water vapor. ($1.006 \times T$)
- Latent Heat: The energy stored in the phase change of the water vapor. ($W \times 2501$)
The Formula
$$h = 1.006 \cdot T_{db} + W \cdot (2501 + 1.86 \cdot T_{db})$$
Where $W$ is the Humidity Ratio (kg water / kg dry air). Note that the Latent Heat term ($W \times 2501$) is often massive compared to the sensible term. Removing humidity (Latent Cooling) requires significant energy.
3. Humidity: Relative vs. Absolute
Relative Humidity (RH) tells you how "full" the air is of water vapor compared to how much it could hold at that temperature. It is useful for human comfort (we feel comfortable between 40-60% RH) and material safety.
Humidity Ratio ($W$), also called Specific Humidity, is the actual mass of water in the air (kg of water per kg of dry air). It is useful for calculations. When air is heated or cooled sensibly (without adding/removing water), RH changes drastically, but $W$ stays constant.
4. The Altitude Effect
Pressure Changes Everything
Standard psychrometric charts are plotted at Sea Level (101.325 kPa). If you are designing for Denver, Johannesburg, or Mexico City, you cannot use a standard chart.
As altitude increases, atmospheric pressure ($P_{atm}$) decreases. For a given temperature and humidity ratio, moist air at high altitude has a higher specific volume (it's less dense). This affects fan sizing and coil performance significantly. This calculator automatically corrects for pressure using the ideal gas relationships.
5. Practical Application: Cooling Load
To calculate the cooling capacity required to cool air from State 1 to State 2:
$$Q_{total} = \dot{m} \times (h_1 - h_2)$$
Where $\dot{m}$ is the mass flow rate of air. This total load ($Q_{total}$) can be split into Sensible and Latent loads:
- Sensible Load ($Q_s$): $1.2 \times L/s \times \Delta T$ (Approx at Sea Level)
- Latent Load ($Q_l$): $3010 \times L/s \times \Delta W$ (Approx at Sea Level)